1. Field of the Invention
This invention relates generally to a gaseous composition useful in plasma etching of semiconductor wafers and the like and to a method of etching semiconductor wafers and the like utilizing the aforesaid composition.
2. Discussion of Background and Prior Art
Plasma etching with directed ions (reactive ion etching) is a well-known technique for making depressions, usually in the nature of slots, in semiconductor wafers and the like. Such slots serve as barriers between adjacent circuit elements providing necessary electrical isolation. The slots are usually filled, for example with amorphous silicon, to provide needed integrity.
Prior to plasma etching of a semiconductor wafer or the like, a masking layer is formed covering the surface of the wafer which is to be etched to a selected depth. Thereafter, a photoresist layer is deposited atop the masking layer. Next, any desired pattern is transferred to the photoresist layer which is then developed to provide a series of troughs in the photoresist layer which expose some portions, but not all, of the masking layer. Thereafter, wet chemical or plasma etching is utilized to eat away the masking layer in the areas of the troughs. This exposes the areas on the surface of the wafer in which it is desired to create depressions or slots.
In carrying out the above process, it is generally desirable to have the masking layer above the wafer be as thin as possible so that any depressions which are to be made in the surface of the wafer are sharply defined by the edges of the masking layer.
Once the needed openings in the masking layer have been formed as described above, the photoresist layer is generally washed away with an appropriate solvent. Thereafter, reactive ion etching is utilized to anisotropically cut desired depressions into the surface of the wafer.
The plasma etch species are directed generally orthogonally towards the surface of the wafer which is being etched. They impinge not only in the depression in the masking layer but also upon the surface of the masking layer. This has an eroding effect upon the masking layer as well as upon the wafer. Generally, the plasma is more selective for etching the wafer than for etching the masking layer, usually by a factor of about five to one. As a result, starting out with a silicon dioxide masking layer of, for example, 1.5 microns thickness, a depression can be formed in the silicon wafer of about 5 microns in depth, while still retaining a silicon dioxide layer of about 0.5 microns at the end of the process. Deeper depressions cannot, however, be formed unless the silicon dioxide layer is made considerably thicker. This, as mentioned above, leads to less delineation of the shape of the depression and is generally undesirable. Yet, in certain instances it is necessary to have relatively deeper depressions than those commonly formed.
Oxygen, in an amount from about 0.5% to about 18%, by mole, has been used along with a majority of a chlorine providing compound, about 5.5% to about 6% silicon tetrachloride or about 25% to about 31% chlorine, in reactive sputter etching of silicon. This process is discussed in the paper "Reactive Sputter Etching of Silicon With Very Low Mask-Material Etch Rates", IEEE Transactions on Electron Devices, Vol. ED-28, No. 11, November 1981 by Christopher M. Horwitz. In essence, the process of this reference leads to silicon dioxide deposition on the walls of slots as the slots are being formed. The amount of the oxygen relative to the amount of the chlorine providing compound is high enough so that silicon dioxide is formed and protects the walls from being etched by chlorine. The deposited oxide masking layer is only very slowly attacked because of the redeposition of silicon dioxide which occurs due to the relatively high amount of oxygen present. The argon non-reactively sputters the silicon at the bottom of the slot and the chlorine reacts substantially only at the bottom of the slot. The result is high selectivity for silicon as opposed to silicon dioxide. However, a substantial and undesirable silicon dioxide layer is formed on the walls of the slot. This must later be removed.